PAR (Protease-activated receptor)-2 is one of the PARs belonging to 7-transmembrane G-protein-coupled receptor family.
Four types of PARs namely PAR-1, PAR-2, PAR-3 and PAR-4 have been cloned so far and these belong to a receptor family mediating the actions of serine proteases such as thrombin and trypsin in various cells. The functions of PAR-1, PAR-2, PAR-3 and PAR-4 have been elucidated as receptors involved in platelet aggregation via thrombin. Structurally or from the standpoint of activation mechanism, PAR-2 has numerous properties common to other PARs. Functionally, however, PAR-2 is never activated by thrombin but is activated by trypsin and tryptase.
A specific site in the N-terminal amino acid sequences of these PARs is cleaved by thrombin or such proteases. Peptide fragments generated by the cleavage bind to the binding site of receptors thereof of themselves to activate the receptors. The amino acid sequences activating the PARs are summarized and expressed according to the one character amino acid expression as below.
PAR-1:SFLLRN-NH2(human)(SEQ ID NO: 1)PAR-2:SLIGKV-NH2(human)(SEQ ID NO: 2)SLIGRL-NH2(mouse)(SEQ ID NO: 3)PAR-3:NonePAR-4:GYPGQV(human)(SEQ ID NO: 4)GYPGKF(mouse)(SEQ ID NO: 5)
PAR-1, PAR-2 and PAR-4 can be activated non-enzymatically by exogenously-added peptides with the amino acid sequences of the peptide fragments generated via the cleavage but PAR-3 cannot be activated by such exogenously-added peptides. Recent research works have verified that mouse PAR-3 is the cofactor of PAR-4 since PAR-3 per se is never activated but is activated in the co-presence of PAR-4 (Nature, 404, 609-613 (2000)).
Cloning of PAR-2 was done in 1994 by Nystedt (Proc. Natl. Acad. Sci. USA, 91, 9208-9212 (1994)). It is known that PAR-2 is activated by tissue factors Factor VIIa and Factor Xa, acrosin as one of sperm proteases, trypsin-like serine protease identified in rat brain, trypsin, tryptase and synthetic peptides of similar sequences as those of PAR-2 ligands (Pharmacological Rev, 53, 245-282, 2001; Br. J. Pharmacol. 1998, 123, 1434-1440).
Furthermore, some PAR-2-activating agents have been reported, which have higher activity than the activity of a partial PAR-2 amino acid sequence (SLIGKV) (SEQ ID NO: 2) activating human PAR-2 and include trans-cinnamoyl-LIGRL-O-NH2 (SEQ ID NO: 6) found by Hollenberg et al. (Br. J. Pharmacol. 1998, 123, 1434-1440) (PNAS, 95, 7766-7771 (1998); BJP, 125, 1445-1454 (1998)).
A report also tells that pharmaceutical agents containing a PAR-2-activating agent as the effective ingredient are useful for the prophylaxis and therapeutic treatment of the decrease of saliva secretion, the decrease of lacrimal fluid secretion or gastrointestinal diseases (Japanese Patent Laid-open Nos. 064203/2001, 181208/2001 and 233790/2001).
However, compounds already reported as PAR-2 agonists are not satisfactory in terms of biological properties, physico-chemical properties, and ready synthesis. Therefore, further examinations have been needed for the development of such PAR-2 agonists into pharmaceutical products.